RO membrane vs pre-filters: what each stage actually does
Five or six cartridges in a row, and most people cannot say which does what. A plain walkthrough, in the order the water meets them.
Follow the water. It enters at one end and comes out drinkable at the other, and each stage removes something specific. Knowing which is which turns a mysterious box into a machine you can look after.
1. Sediment filter
A pleated or spun cartridge that catches grit, sand, rust and silt. Purely mechanical. It protects everything downstream from physical damage. Cheapest stage, changed most often.
2. Carbon pre-filter
Activated carbon that adsorbs chlorine, some organics and odours. Its main job is protecting the membrane, which chlorine degrades. If it is spent, the membrane ages fast.
3. RO membrane
The heart of it. A tightly wound sheet that lets water molecules through under pressure and rejects most dissolved salts, heavy metals and microorganisms. This is where TDS drops. It is the expensive part and the slow part.
4. Post-carbon filter
A final polish for taste, removing anything the water picked up sitting in the tank. This is why water straight from a new post-carbon filter tastes clean.
5. UV or UF, if fitted
UV kills bacteria and viruses with ultraviolet light; UF is a fine physical barrier. Either is a safety net after the membrane, particularly useful if the tank sits for long periods.
6. Mineraliser or TDS controller, if fitted
Adds a controlled amount of minerals back, or blends a little filtered-but-not-RO water in, so the result is not flat.
The order is not arbitrary. Swapping a sediment and carbon cartridge, or fitting a post-carbon in a pre-carbon slot, is a common DIY mistake that quietly ruins a membrane.
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